Hmm... See, I've been wanting a Zelda/Sonic crossover for a while. Ha ha, yeah, what a surprise. But no seriously, think about it. Demise escapes to Mobius, posesses Eggman and gives him magics. The goddesses give the Triforce to Sonic, Tails and Knuckles, and the Hyrulian holders' spirits are sent in the Triforce pieces to be their helper fairies. Yes, Knuckles has to deal with Ganindorf. That's a plot point. Anyway, Hyrule monsters and stuff start filling Mobius, terrorizing people and all, and turning old bases into Sonic-y Temples. Sonic, Tails and Knuckles go through these in a sword-fight, magic-y running sort of way, a good mix between Zelda and Sonic playstyles, and get the Chaos Emeralds. They exorcise Eggman with them, but Demise just posesses Knuckles. With him and Ganondorf in the echidna's head, he turns evil and kidnaps Tails, for both Tails and Zelda reasons. Sonic then has to find the Seven Sages, who are all more minor Sonic characters, and go through more temples and stuff before finally having the power to banish Demise back to Hyrule and save Mobius from this otherworldly threat.
Boom. Nintendo, Sega, make it bleeping happen.